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338
CINEMATOGRAPHIC ANNUAL
only approximately seventy-five times. This being due to the fact that theatrical films are projected for simultaneous viewing by several hundred, even a few thousand people, while 16 millimeter films are usually shown to a much more limited audience.
The necessity of a much greater illumination for the first is obvious, and the source of light usually used in the carbon arc, though incandescent lamps of high wattage have been used with good success.
For the 16 millimeter film incandescent lights have been found amply sufficient with regard to illumination and much more recommendatory because of the elements of safety and greater ease of operation.
A two hundred and fifty watt lamp is amply sufficient for home use where an excess of screen brilliancy would perhaps result fatiguing to the eye.
For the projection of semi-professional films for educational, industrial or commercial use, lamps of higher intensities can be advantageously used, and lamps burning at low voltage and high amperage have been designed for this particular purpose. Better still, the Bell & Howell Company has put on the market a three hundred seventy-five watt lamp which offers the possibility of a brighter screen image than heretofore attainable, and which, through a variable resistance, can be made to burn at any desired wattage, thus affording the possibility of regulating the brilliancy of the screen according to the condition of projection, the length of the throw and the density characteristic of the film.
Fig.
4. A — Reflected filament images properly focused and intermeshed. B — Filament intermeshed but badly out of focus. C — Filament property focused but not intermeshed.
The crater of a carbon in an arc light, which is the main source of illumination in professional projection, produces a relatively small surface of intense brilliancy and the condenser system is so designed and placed in the projection machine that all of the rays of light, those directly incident upon the condenser and those reflected by the refractor usually used in those lamps, fill the condenser and are brought to an intensely illuminated spot fully covering the projector aperture.
The same attributes are essential in 16 millimeter projection. Furthermore, the fact that the filaments of an incandescent lamp